This invention relates to a new polymerization catalyst system comprising an aluminum compound and a transition metal compound on an alumina-based aerogel support, a process for preparing the polymerization catalyst system and use of the polymerization catalyst system for polymerization and copolymerization of alpha-olefins. Another aspect of this invention relates to a heat-activated alumina-based aerogel useful as a catalyst support and having a morphology by transmission electron microscopy comprising film, platelets and needles and substantially free of spherical particles and having a high BET surface area, high pore volume, and low bulk density.
It is well known that alpha-olefins may be polymerized and copolymerized in the presence of a Ziegler-Natta type catalyst comprising Group III metal compound such as Al(C.sub.2 H.sub.5).sub.3 and a transition metal compound such as titanium tetrachloride on an inorganic oxide support such as alumina, silica, titania, magnesia, etc. The polymerization reaction may be carried out in suspension, in solution or even in the gas phase. (See, for example, Professor Natta's article in Encyclopedia of Polymer Science and Technology, Volume 4, at pages 137 to 150, (1971) J. Wiley & Sons, Inc., and articles in Volume 13, at pages 13 to 122 and Volume 15, at page 133 ibid.
U.S. Pat. No. 3,506,633 (Matuura, et al.) discloses a polymerization catalyst having a Cl/Ti ratio of 2.5 to 3.5 that is prepared by reacting TiCl.sub.4 with a substantially amorphous alumina xerogel having a total pore volumn less than 0.7 cm.sup.3 /g.
U.S. Pat. No. 3,978,031 (Reginato, et al.) discloses a polymerization catalyst system containing an organo-metallic compound such as an alkyl aluminum compound and a co-catalyst formed by reacting a heat-activated halogenated alumina having an atomic ratio of halogen to aluminum of from 0.1 to 1, such as fluoronated alumina, and a transition metal compound such as TiCl.sub.4.
U.S. Pat. No. 4,088,812 (Matuura, et al.) discloses preparation of an olefin polymerization catalyst by impregnating a titanium or a vanadium compound such as TiCl.sub.4 onto a solid carrier formed by treatment with SO.sub.3 of an oxide or mixture of oxides of Group II-IV metals such as alumina.
U.S. Pat. No. 4,247,669 (Reginato, et al.) discloses an olefin polymerization catalyst system containing an organo-metallic compound such as trialkyl aluminum and a supported catalyst prepared by reaction of a halogen-containing transition metal compound such as TiCl.sub.4 with a heat-activated alumina having an internal pore volume greater than 0.8 cm.sup.3 /g so that the ratio of halogen to transition metal in the supported catalyst is greater than that of the halogen-containing compound.
All of the above-mentioned U.S. patents disclose polymerization catalysts that are characterized by relatively low productivity in the low pressure (&lt;1000 psi) polymerization of ethylene. In commercial production of ethylene, the use of catalysts having a high productivity (which is a measure of the grams of polymer produced per gram of catalyst per hour) is frequently the difference between making an acceptable or a non-acceptable product. The higher the catalyst productivity, the lower the concentration of catalyst remaining in the polymer. Very low concentrations of catalyst residue in the polymer are innocuous and, consequently, need not be removed by expensive de-ashing procedures. For this reason, the polyolefin industry has ongoing research efforts on developing polymerization catalysts having high productivity for the low pressure polymerization of ethylene.
An inorganic hydrated oxide, precipitated from an aqueous solution of the corresponding metal cation washed and then dried in an oven (in air or in vacuum) is very often obtained in a divided state as a porous gel. The general name of xerogel is given to these materials by A. Freundlich (Colloid and Capillary Chemistry), Duttom, N.Y. 1923). However, the texturial characteristics (pore volume and surface area) of the xerogel is considerably poorer than that of the wet gel before the elimination of the solvent (water). It is theorized that the evaporation of the solvent creates a vapor-liquid interface inside the pores and that the surface tension of the solvent is responsible for a partial collapse of the pore structure. In order to eliminate the liquid-vapor interface inside the pores, Kistler (J. Phys. Chem., 36 (1932) 52) disclosed an efficient process of evacuating the solvent from the system under supercritical conditions in an autoclave. The general name of aerogel is given to solids dried in this way. S. J. Teichner et al. (article entitled "Inorganic Oxide Aerogels" in Advances in Colloid and Interface Science, Volume 5, 1976) 245-273) disclosed a general method for preparation of inorganic oxide aerogels such as SiO.sub.2,Al.sub.2 O.sub.3,TiO.sub.2,ZrO.sub.2,MgO and mixed inorganic oxides by dissolving in an organic solvent such as alcohol or benzene the corresponding alcoholate of the metal, hydrolyzing same at room temperature and evacuating the solvent under supercritical conditions in an autoclave. The method disclosed by Ieichner is simpler than the complicated method of Kistler in that the hydrolysis reaction is carried out directly in an organic medium such as alcohol or benzene and there is no need for the substitution of an organic solvent for the initial aqueous medium which was previously used in the preparation of aerogels. U.S. Pat. No. 3,963,646 (Teichner et al.) disclosed preparation of NiO-Al.sub.2 O.sub.3 aerogels useful as catalysts for the hydrogenation or the controlled oxidation of olefins. See also M. Astier et al. in Preparation of Catalysts, edited by B. Delmor et al., Elsevier Scientific Publishing Company (1976) Amsterdam, at pages 315 to 328.
U.S. Pat. No. 4,018,672 (Moser) discloses a hydrodesulfurization catalyst having an alumina-containing support prepared by a thermal decomposition of aluminum alcoholates in a manner analagous to that disclosed by Teichner et al.